During conventional radiation therapy treatment, radiation beams of varying angles and intensities are directed at a target in a patient. Normal tissue and organs located in the path of the radiation beams must be taken into account for safety reasons, thereby limiting the dose that can be delivered to the target. Many techniques are known for shaping the radiation beams so that the radiation is concentrated at the target and is minimized or eliminated at the normal tissues. One of the techniques is conformal radiation therapy wherein the beam aperture varies from angle to angle via a multileaf collimator which employs a multiplicity of radiation blockers, called leaves. Each individual leaf in a multileaf collimator can be positioned independently, allowing the user to create an infinite amount of irregularly shaped fields. The radiation beams are directed between the ends of opposing arrays of the radiation blocking collimator leaves, thereby shaping the beam to closely match the shape of the desired treatment area, while shielding the normal tissue and organs.
An example of such a system is U.S. Pat. No. 5,166.531 to Huntzinger which describes a multileaf collimator arrangement positioned about the central axis of a radiation emitting head for shaping an emitted radiation beam. The collimator includes two opposing arrays of side-by-side elongated radiation blocking collimator leaves. Each leaf in each opposing array can be moved longitudinally towards or away from the central axis of the beam, thus defining a desired shape through which the radiation beam will pass. However, because the adjoining leaves must be tightly positioned side-by-side in order to minimize radiation leakage between the leaves, friction is an inherent problem, creating complications in maintaining a set position of one leaf while re-positioning an adjacent leaf, such repositioning being frequently required in conformal therapy. If friction between the adjacent leaves is reduced by providing a looser fit between adjacent leaves, unacceptable radiation leakage through spaces between the adjacent leaves will result. On the other hand, maintaining a tight leaf fit between the adjacent leaves and providing a lubricating layer in the contact area of the adjacent leaves is also not an acceptable solution because the lower density of the lubricating layer, as compared to the high density of the collimator leaves, will allow an unacceptable amount of radiation leakage to occur.
U.S. Pat. No. 5,591,983 to Yao, the disclosure of which is incorporated herein by reference, attempts to solve the friction problem by providing a collimator that comprises first and second layers of a plurality of elongated radiation blocking leaves. The leaves of each layer are arranged adjacent one another so as to form two opposed rows of adjacently positioned leaves and are movable in a longitudinal direction (Y) which is generally traverse to the direction of the beam so as to define a radiation beam shaping field between the opposed ends of the leaves. The layers are arranged one above another in the beam direction and offset in a lateral direction (X) generally transverse to the beam direction and orthogonal to the longitudinal direction (Y) so that spaces between adjacent leaves of the first and second layers are positioned over and under, respectively, leaves of the second and first layers, respectively.
One disadvantage of the Yao system is that an irregularly shaped target is poorly covered by the two layers of leaves. In addition, the overall thickness of the leaves in the beam direction is quite large.
Other patent documents related to multileaf collimators include U.S. Pat. Nos. 4,233,519 to Coad, 4,534,052 to Milcamps, 4,672,212 to Brahme, 4,739,173 to Blosser et al., 4,754,147 to Maughan et al., 4,794,629 to Pastyr et al., 4,868,843 and 4,868,844 to Nunan, 5,012,506 to Span et al., 5,165,106 to Barthelmes et al., 5,207,223 to Adler, 5,343,048 to Paster, 5,351,280 to Swerdloff et al., 5,427,097 to Depp, 5,438,991 to Yu et al., 5,442,675 to Swerdloff et al., and 5,555,283 to Shiu et al.